The outgoing UN envoy to Syria failed to bring peace to the war-torn country because of his trust in the Syrian regime's willingness to participate in political negotiations, the opposition spokesman told The National.

Yahya Al Aridi said on Tuesday Staffan de Mistura should have ended his mission years ago, upon realising President Bashar Al Assad's resistance to a non-military settlement.

"He was highly experienced and patient with the warring sides," said Mr Al Aridi. However, "De Mistura should have said that the regime made his mission impossible but he didn't and that cost Syrians blood and devastation".

After four years as Syria's chief mediator – tasked with bringing the country's warring sides together – Mr de Mistura announced last week he would soon step down, citing personal reasons.

The move follows a series of military victories by Mr Al Assad's military, backed by Iran and Russia, and the loss of rebel territory.

Meanwhile European leaders, along with Turkish President Recep Tayyip Erdogan and his Russian counterpart Vladmir Putin, are expected to hold a summit on the Syria on Saturday.

The talks aim to identify the best ways to avoid a humanitarian catastrophe in the northern province of Idlib, by ensuring a ceasefire between the regime and rebel forces remains in place.

Mr Erdogan has on three occasions attended trilateral summits on Syria with Mr Putin and Iranian President Hassan Rouhani. But this is the first time a meeting will bring the Turkish and Russian strongmen together with the EU's two most significant leaders, Chancellor Angela Merkel and President Emmanuel Macron.

The ceasefire, known as the Idlib agreement, was brokered by Ankara and Moscow and announced in September. It aims to create a demilitarised buffer zone ringing the Idlib region, home to three million people and sought to avoid an attack by Mr Al Assad and the mass exodus of civilians across the Turkish border.

According to the deal, the zone would separate rebel and regime zones under the supervision of the two sponsor countries.

"This summit aims to support the Sochi agreement on Idlib. It needs to be solidified, and the attempts by the regime to sabotage it should stop," Mr Al Aridi said.